Anatomy and Physiology of Swallowing

Original Editor - Srishti Banerjee

Top Contributors - Ewa Jaraczewska and Jess Bell  

Introduction

Deglutition, or swallowing, refers to the movement of liquids or solids from the mouth to the stomach via the pharynx and oesophagus. Swallowing starts to develop in utero from around 15 weeks gestational age. It continues to develop after birth, ultimately resulting in the conscious control of swallowing. Multiple organ systems are involved in swallowing, including the musculoskeletal, neuromuscular and respiratory systems. Normal swallowing requires a coordinated effort of over thirty muscles, the central nervous system, and five cranial nerves. Other key anatomical structures involved in this process are the nose, nasal cavity, oral cavity, and pharynx. This article explores the relevant anatomy and physiology of normal swallowing.

Developmental Anatomy and Swallowing

There are a number of anatomical changes that occur from infancy to childhood.

Infant:

• teeth have not erupted
• the hard palate is flatter
• larynx and hyoid bone are located high in the neck
• the epiglottis touches the back of the soft palate
• the airway is separated from the oral cavity by a soft tissue barrier
• jaw, oral cavity and larynx are small
• tongue occupies a significant portion of the mouth
• uses tongue, sucking pads, and sulci to suck
• sucking pads provide stability
• lack of a distinct oropharynx
• the base of the tongue is close to the larynx

Older child:

• the mouth is bigger, so the tongue lies at a lower position (floor of the mouth) and sits behind the teeth
• the mandible extends down and forward
• the oral cavity expands
• the neck gets longer, so the larynx descends to a lower position in the neck
• the contact between the epiglottis and soft palate is gradually lost
• the pharynx lengthens vertically
• the hyoid descends
• the sucking pads degenerate

Note: these anatomical changes are essential for speech, but they make humans more prone to aspiration.

Adult Anatomy and Swallowing

Muscles

Table 1 lists the major muscles of swallowing and their function. Some muscles are listed more than once in the table to highlight their relationships with different muscles and structures.

Table 1. Muscles of swallowing

	Muscles	Function
Tongue muscles	Intrinsic tongue muscles: Hyoglossus Geniohyoid Genioglossus Styloglossus	Hyoglossus: depresses and retracts the tongue Geniohyoid: pulls the hyoid bone up and forward during swallowing for effective bolus flow and pharyngeal clearing Genioglossus: maintains upper airway patency by tongue protrusion and stiffening the tongue prevents posterior tongue displacement and upper airway closure Styloglossus: retracts and elevates the tongue
Masticatory muscles	Mylohyoid Masseter Temporalis Medial and lateral pterygoid muscles	Mylohyoid: mastication, swallowing and speech production: works directly and indirectly with the infrahyoid muscles on the position of the hyoid bone during swallowing, when the mandible is fixed, it elevates the base of the tongue and the hyoid bone in an anterosuperior direction when the hyoid bone is in a fixed position, it depresses the mandible and elevates the oral cavity Masseter: elevates and protracts the mandible Temporalis: elevates and retracts the mandible Medial and lateral pterygoid: lateral pterygoid: a bilateral contraction protrudes and depresses the mandible; a unilateral contraction causes the lateral pterygoid to move the mandible medially medial pterygoid: a bilateral contraction elevates, closes and protrudes the mandible; a unilateral contraction causes swinging of the mandible to the opposite side
Neck muscles	Anterior belly of digastrics (ABD) Posterior belly of digastrics (PBD)	ABD: stabilises the hyoid during swallowing, which helps protect the airway while eating PBD: aids in head extension ABD and PBD lift the tongue base and hyoid bone and depress the mandible for jaw opening, chewing, and speech
Soft palate and pharynx	Tensor veli palatini (TVP) Levator veli palatini (LVP) Palatopharyngeous (PPh) Salpingopharyngeous (SaPh) Stylopharyngeus (SPh) Intrinsic laryngeal muscles (IL)	TVP: tenses the soft palate LVP: elevates the soft palate PPh: tenses the soft palate and pulls the pharyngeal walls superiorly, anteriorly, and medially during swallowing, and effectively closes off the nasopharynx from the oropharynx SaPh: raises the pharynx and larynx during swallowing and laterally draws the pharyngeal walls up SPh: elevates the pharynx and larynx, IL: open and close the vocal folds, lengthen and shorten the vocal folds
Upper oesophageal sphincter	Extrinsic muscles anterior group: suprahyoid muscles (geniohyoid, mylohyoid, stylohyoid, hyoglossus and anterior belly of digastric) infrahyoid muscles (thyrohyoid, sternohyoid, sternothyroid and omohyoid) posterior group: stylopharyngeus, palatopharyngeus, and pterygopharyngeus Intrinsic muscles: cricopharyngeus muscle (CPM) inferior constrictor cranial aspect of the circular oesophageal muscle	Extrinsic muscles: anterior group: suprahyoid muscles move the hyoid bone anterosuperiorly please note that there are variable descriptions of the function of stylohyoid in the literature infrahyoid muscles thyrohyoid and omohyoid pull the larynx up and anteriorly with the hyoid bone sternohyoid and sternothyroid stabilise the larynx during swallowing posterior group: shorten the pharynx, pulling the upper oesophageal sphincter upwards while also widening it Intrinsic muscles: prevent the swallowing of air during respiration and phonation and the regurgitation of oesophagal contents into the airway CPM relaxes and allows food to pass into the oesophagus

Figure 1. Cranial nerves

Cranial nerves

Cranial nerves modulate the execution of swallowing. Table 2 summarises the function and the clinical relevance of the six cranial nerves involved in swallowing.

Table 2. Cranial nerves involved in swallowing

Cranial nerves (CN)	Function	Clinical Relevance
Trigeminal nerve (TN) CN V	Controls somatosensation of the face and the anterior two-thirds of the tongue Provides motor innervation of the following muscles: mylohyoid, anterior belly of the digastric muscle, masticatory muscles, and the tensor veli palatini	Impairment of the TN can lead to: problems in the preparatory oral and oral phase of swallowing due to poor mastication and poor stabilisation of the mouth floor decreased hyolaryngeal excursion during the pharyngeal phase of swallowing due to mylohyoid and anterior belly digastric muscle impairment
Facial nerve (FN) CN VII	Conveys taste sensation in the anterior two-thirds of the tongue Controls the motor movement of the orbicularis oris and buccinator muscle, which are involved in the closure and prevention of oral residue Provides motor innervation of the stylohyoid muscle and the posterior belly of the digastric muscle Innervates the submandibular and sublingual salivary glands	Impairment of the FN can result in: decreased taste perception poor bolus formation during the preparatory oral phase anterior bolus spilling post-swallow oral residue dry mouth
Glossopharyngeal nerve (GN) CN IX	Controls somatosensation of the posterior third of the tongue, the mucosa of the soft palate and the upper pharyngeal tract Provides autonomic innervation of the parotid gland Provides motor innervation of the stylopharyngeus muscle, which assists in the opening of the upper oesophagal sphincter	Dysfunction of the glossopharyngeal nerve can result in: impaired pharyngeal bolus transport impaired upper oesophagal sphincter opening, resulting in post-swallow pharyngeal pooling
Vagus nerve (VN) CN X	Provides motor innervation to all striated muscles of the larynx and pharynx, except the stylopharyngeus and the tensor veli palatini The pharyngeal branches of the vagus nerve innervate the levator veli palatini, salpingopharyngeus, palatopharyngeus, palatoglossus, and the uvular muscle The external superior laryngeal nerve (SLN) supplies the motor innervation of the cricothyroid muscle The recurrent laryngeal nerve (RLN) is responsible for the motor innervation of all intrinsic laryngeal muscles except for the cricothyroid muscle The RLN and the SLN internal branches provide mucosal sensory innervation of the pharynx, larynx, and proximal trachea	Impairment of the VN can cause: poor velopharyngeal seal and nasal reflux weak pharyngeal contraction reduced vocal fold adduction, resulting in dysphonia and poor cough effectiveness impaired upper oesophageal sphincter opening—post swallow pharyngeal pooling silent aspiration
Hypoglossal nerve (HN) CN XII	Innervates all intrinsic and extrinsic tongue muscles, except for the palatoglossus muscle Controls all movements of the tongue	Impairment of the HN can cause: dysarthria problems with oral control of the bolus problems with bolus propulsion due to poor lingual pressure premature posterior spill of the bolus to the pharynx post-swallow oral residue
Accessory nerve CN XI	Provides motor innervation to the striated portions of the pharynx, larynx and oesophagus in association with the vagus nerve Parasympathetic fibres (general visceral efferent) accompany the vagus nerve fibres The pressure transferred from the oral cavity to the pharynx by distention produces afferent stimuli to the brainstem, which generates efferent motor stimuli to the pharyngeal structures. The accessory nerve prevents pressure from returning to the oral cavity Elevates the palate and blocks possible pressure escape from the oropharynx to the rhinopharynx	Impairment of the accessory nerve can cause: reduced adduction of the vocal folds

Anatomical Structures

"Feeding and breathing share the same anatomy."

Nose and nasal cavity: we breathe through our nose while eating solids since our is mouth processing the food, and our lips are sealed to prevent food from escaping anteriorly. Nasal air pressure oscillates with masticatory jaw movement. It becomes positive relative to atmospheric pressure during jaw closure and negative during jaw opening.

Oral cavity: tongue movement corresponds with cyclic jaw movement when food is in the mouth. The tongue repositions food laterally and the cheeks reposition food medially.

Pharynx: a breathing, mastication and swallowing route. The pharynx dilates to maintain the airway for breathing and constricts to push the food bolus to the oesophagus for swallowing.

Larynx and vocal folds: the posterior aspect of the larynx forms the anterior wall of the upper oesophageal sphincter.

Upper oesophageal sphincter: a fibromuscular structure located behind the larynx. It is bordered posterolaterally by the pharyngoesophageal muscles and superiorly by the pharynx. Inferiorly, it continues into the oesophagus.

Neural Coordination of Swallowing

Swallowing requires a coordinated contraction of the muscles in the mouth, pharynx, upper oesophageal sphincter, and upper oesophagus via central control. Swallowing centres activate the voluntary motor centres and inhibit the respiratory centres - this prevents food from entering the trachea. There is also activation of the:

• reflex centres
• nuclei of the cranial nerves that are involved in the movement of the tongue, larynx and pharynx

Please see the swallowing flow chart in Figure 2 for more information.

Figure 2. Swallowing flow chart.

Figure 3. Physiology of swallowing.

Physiology of Swallowing

Swallowing is often described as having four phases: (1) oral preparatory phase, (2) oral propulsive phase, (3) pharyngeal phase, and (4) oesophageal phase. The oral phases change depending on whether we are drinking liquids or eating solids.

Swallowing Liquids

Oral Preparatory Phase

The oral preparatory phase starts as soon as you take a sip. It is a voluntary phase of swallowing, and includes the following steps:

• formation of a liquid bolus in the mouth
• holding the bolus in the anterior part of the mouth, known as the anterior floor of the mouth, or on the surface of the tongue against the hard palate
• the tongue and the soft palate seal the oral cavity posteriorly to prevent leakage of the liquid into the oropharynx

Oral Propulsive Phase

The oral propulsive phase immediately follows the oral preparatory phase:

• the tip of the tongue rises to touch the hard palate
• as the tip of the tongue rises, the oral cavity opens
• the tongue-to-palate contact area gradually widens in an anterior-posterior direction
• the bolus is pushed backwards along the pharynx

To prevent liquid aspiration, "the bolus is commonly held in the oral cavity until just before the initiation of the pharyngeal swallow." With solids, the food is propelled to the oropharynx, accumulated, and bolus is formed before swallowing.

Swallowing Solids

The Process Model of Feeding describes the oral stage of eating solid food.

Oral Preparatory Phase

Stage I Transport

• after food enters the mouth, the bolus is taken to the post-canine dentition, where it is rotated laterally

Food Processing

• mastication (i.e. chewing) reduces the food particle size
• saliva softens the food, making it an appropriate consistency for swallowing
• there are cyclical movements of the jaw, with movements of the tongue, cheek, soft palate, and the hyoid bone
• unlike with liquids, the posterior cavity isn't sealed - the cyclical movement of the jaw and soft palate allows air to be pumped into the nasal cavity through the pharynx; this delivers the aroma of the food to the chemoreceptors in the nose
• movements associated with chewing include:
  • early jaw opening: the lips are sealed, and the tongue and jaw move forward and downward
  • late jaw opening: the lips are open, and the tongue curls (this prevents biting of the tongue)
  • the tongue moves mediolaterally and rotates along its antero-posterior axis
  • the hyoid bone has an important role in controlling jaw movements, acting as a stabiliser for the jaw and the tongue

Clinical relevance: Older adults have increased chewing cycles and activity of the jaw adductor muscles during mastication, which increases mastication time. However, despite this increase, the bolus size at the time of swallow onset is larger in older adults than in younger people because of a decrease in masticatory function.

Oral Propulsive Phase

Stage II Transport

• when the bolus of food is ready to be swallowed, it is placed on the surface of the tongue and is propelled backwards to the oropharynx
• chewing continues while food remains in the oral cavity

Clinical relevance: "The initial consistency of food affects the duration of oropharyngeal bolus aggregation before the swallow and the number of chewing cycles. There are more chewing cycles, and the oropharyngeal bolus aggregation time is longer for hard foods than for soft foods.

Pharyngeal Phase

During the pharyngeal phase, there are two key events:

• food passage: the bolus is propelled from the pharynx to the upper oesophageal sphincter to the oesophagus
  • the tongue elevates and pushes the bolus against the wall of the pharynx
  • tongue elevation blocks the oral cavity
  • the pharyngeal constrictor muscles contract, creating a peristaltic wave
  • the bolus is pushed in the downward direction
• airway protection: the epiglottis, arytenoids, and vocal folds are the three gatekeepers to prevent aspiration:
  • the soft palate elevates and comes into contact with the lateral and posterior walls of the pharynx, which closes the nasopharynx
  • the elevation of the soft palate prevents the regurgitation of food
  • the vocal cords close and seal the epiglottis
  • the transverse arytenoid muscles tilt forward to contract the epiglottic base and seal the epiglottis
  • the suprahyoid and thyrohyoid muscles contract and pull the hyoid bone up
    • as the larynx is attached to the hyoid bone, it moves up and forward following this muscle contraction
    • as a result, the backward tilting of the epiglottis seals the laryngeal vestibule

All the above activities must occur before the upper oesophageal sphincter opens because once it does open, the bolus will pass down to the oesophagus.

In the next step of the swallowing process, the upper oesophageal sphincter opens, and the bolus is transported forward. For the upper oesophageal sphincter to open, the following events must occur:

• relaxation of the cricopharyngeus muscle
• contraction of suprahyoid and thyrohyoid muscles which pulls the hyrolaryngeal complex forward, leading to the opening of the upper oesophageal sphincter
• pressure created by the descent of the bolus helps to open the upper oesophageal sphincter

Oesophageal Phase

The oesophagus is a tube-like structure that has three anatomical segments: cervical, thoracic and abdominal. It extends from the upper oesophageal sphincter to the lower oesophageal sphincter. The top third of the oesophagus is largely made up of striated muscles, while the bottom two-thirds is smooth muscle.

The oesophageal phase includes the following activities:

• food passes through the upper oesophageal sphincter to the oesophagus
• a peristaltic wave develops, which transports the bolus through the oesophagus
• during swallowing, the lower oesophageal sphincter relaxes to allow the bolus to enter the stomach - it is contracted at rest to prevent regurgitation of food from the stomach

This optional video discusses the anatomy and physiology of swallowing:

Swallowing and Respiration

Swallowing and breathing are coordinated actions. Swallowing usually starts during the expiratory phase of breathing. During swallowing, breathing briefly pauses because the airway is physically closed by the elevation of the soft palate and tilting of the epiglottis. There is also neural suppression of respiration in the brainstem. This pause continues for 0.5-1.5 seconds during swallowing, before respiration resumes with expiration after swallowing.

This sequence of exhale-swallow-exhale prevents the inhalation of any food that remains in the pharynx. Other patterns of swallowing and respiration include (1) “inhale-swallow-exhale”, (2) “exhale-swallow-inhale” and (3)“inhale-swallow-inhale”. These patterns occur rarely in healthy adults.

Note: "When performing sequential swallows while drinking from a cup, respiration can resume with inspiration." During mastication, the respiratory cycle duration decreases and then increases with swallowing following the sequence of exhale-swallow-exhale.

Resources

• Hao N, Sasa A, Kulvanich S, Nakajima Y, Nagoya K, Magara J, Tsujimura T, Inoue M. Coordination of Respiration, Swallowing, and Chewing in Healthy Young Adults. Front Physiol. 2021 Jul 13;12:696071.
• Matsuo K, Palmer JB. Coordination of Mastication, Swallowing and Breathing. Jpn Dent Sci Rev. 2009 May 1;45(1):31-40.
• Feeding and Swallowing Development in Children